Chemical and self-assembly approaches are increasingly used for the fabrication of nanostructured materials with controlled optical properties.We have considered the simplest case of composite films comprised of a non-structured polymer with spherical gold nanoparticle inclusions, with varied thickness and filling fractions. Optical properties have been studied by means of spectroscopic ellipsometry in the ultraviolet-visible-near infrared range. For thick, three-dimensional films (30-200 nm) with filling fractions below 6%, the behavior of the optical index as a function of the wavelength is well described using a modified Maxwell-Garnett mixing law modified with anisotropic polarisabilities, which accounts for electromagnetic couplings between neighboring particles. Very dense samples with filling fractions up to 30% have also been fabricated, and exhibit near-zero or negative electric permittivities in a finite frequency range. For very thin films (of thickness close to the nanoparticle size), we have shown that the complex optical index is not an intrinsic descriptor of the system and that ellipsometric analysis cannot determine a sample thickness unambiguously. However, the analysis gives access to the particles extinction cross-section : we find that when the particles lie very close to the silicon substrate, experimental values are enhanced compared to theoretical predictions, even taking into account image dipole effects within the substrate. This work has shed light on the relation between the optical properties and the nanostructure of composite films, and opens the way for the fabrication of nanocomposite materials with novel optical properties via chemical routes.
